The present invention relates to a torque transmission device such as a brake or clutch. It relates in particular, though not exclusively, to a torque transmission device in the form of a multi-disc brake assembly for an aircraft wheel and to a wheel and brake assembly comprising said torque transmission device.
An aircraft brake assembly typically comprises a torque tube having a splined outer surface on which is mounted a series of stator type friction discs of a material such as steel or carbon. The stator discs are interleaved with rotor discs carried by the aircraft wheel and the interleaved assembly of discs is axially compressible between a series of circumferentially spaced actuation pistons secured to one axial end region of the torque tube and a thrust reaction member at the other end region of the torque tube.
The torque tube is coaxial with a wheel axle relative to which a wheel is rotatably mounted by means of bearings secured to the axle, usually in an annular space between the radially outer surface of the axle and the inner surface of the torque tube.
The torque tube commonly includes a so-called pedestal which extends radially inwards to a foot portion that is supported by the radially outer surface of the axle.
In the aforedescribed construction of a typical aircraft brake assembly the torque tube locates the stator discs, provides a thermal barrier between the brake friction discs and the axle, increases whirl stability and enhances vibration damping in addition to the structural function of transmitting torque during a braking operation.
During a braking operation the torque transmitted by the torque tube can result in a change in the annular configuration of that tube. The torque tube can experience some consequential whirl instability and torsional movement and can also experience squeal vibrations. It is recognised by brake designers that it would be preferable to achieve an improvement in the precision with which the configuration of the torque tube is maintained during braking operations so as to improve whirl stability, increase torsional damping and reduce squeal vibrations.
One proposal for endeavouring to improve the maintenance of the torque tube configuration during braking operations is described in the specification of U.S. Pat. No. 5,485,898 (Patko). That proposal comprises provision of a spacer member located radially between the torque tube and axle, at a position axially outwards of the conventional pedestal, so as to lie axially between the pedestal and wheel bearings.
The pedestal typically is integral with the torque tube and extends radially inwardly for support by bushing means associated with the axle. The spacer member is located axially between a radially inner region of that pedestal and the wheel bearings.
The outer diameter surface of the spacer member is provided with a coating such as aluminium bronze which contacts and supports the inner diameter surface of the torque tube. The coating may serve to provide improved frictional contact between the spacer member and inner diameter surface of the torque tube. Additionally a support ring is disposed between the inner diameter surface of the spacer member and the outer diameter surface of the axle, the support ring and spacer member each including respective splines to effect a splined interconnection which prevents relative rotation of the ring and spacer member. In this construction it is claimed by the prior art teaching that the provision of the spacer member leads to improved torque tube configuration stability and support.
It has now been found that notwithstanding the alleged benefits of a spacer member and coated surface there are problems due to fretting and/or galling wear at the contact areas between the spacer member and adjacent surfaces. Small movements at those interfaces lead to wear and in consequence yet further interface movement during braking operations. As that wear, and hence relative movement, increases there is a progressive reduction in the beneficial effects which the spacer member might otherwise provide.